Photodiode arrays are generally used as a photodetector in a spectrophotometer or other apparatuses since they are capable of collectively receiving a spectrum of light dispersed by a diffraction grating or similar device, thus almost simultaneously detecting the intensity of light over a wide range of wavelengths.
In a photodiode array, each photodiode generates a current, voltage or another form of signal corresponding to the intensity of light falling on it. The signals of the photodiodes are then sequentially read out and processed one after another by a signal-processing circuit. There are two major methods for separately reading out signals from the photodiodes: the real-time readout method and the storage readout method.
In the real-time readout method, one amplifier is provided for each photodiode, and the photodiodes are connected to the signal-processing circuit via a multiplexer. This system can selectively read out a photocurrent generated in each photodiode as a result of illumination with light. However, this system has a drawback in that its signal-to-noise (S/N) ratio is difficult to increase since the signal intensity depends exclusively on the intensity of the incident light. Therefore, if the light intensity is low, it is necessary to amplify the signal by an operational amplifier.
In the storage readout method, the value of the read signal is proportional to the product of the light intensity and the illumination time of the photodiode. FIG. 1(a) is a circuit diagram of a storage readout system, showing the section from the photodiodes 11 to the signal-processing circuit 15. Before an illumination with light, a specific amount of electric charge is stored in the photodiode (PD) junction capacitor 12, as indicated by the chained line in FIG. 1(b). When illuminated with light, the photodiode 11 generates a photocurrent, which causes the electric charge to be discharged from the PD junction capacitor 12, thus decreasing the amount of the stored electric charge. Subsequently, the PD junction capacitor 12 is connected via the multiplexer 14 to the signal-processing circuit 15 so that the same circuit 15 can read the reduced amount of the electric charge remaining in the PD junction capacitor 12. Simultaneously, the PD junction capacitor 12 is charged to the level of the junction capacitance, i.e. to the level indicated by the chained line in FIG. 1(b). After the connection to the signal-processing circuit 15 is terminated by the multiplexer 14, the stored electric charge is discharged once more from the PD junction capacitor 12 due to the photocurrent until the next cycle of readout operation (i.e. during the readout period). In other words, the readout period corresponds to the period of time for the PD junction capacitor 12 to be discharged, and the read signal is proportional to the product of the light intensity and the readout period. This means that, in the storage readout system, a sufficiently strong signal can be produced by increasing the readout time if the light intensity is low.
The present invention specifically relates to a photodiode array that reads out signals by a storage readout method. Accordingly, the following description presumes the use of the storage readout method.
In actual cases, the operation of reading out signals from the photodiode array is performed by the sequential access method, in which a signal is read out from each photodiode in a predetermined order (e.g. from shorter to longer wavelengths). This method generally uses shift registers.
The photodiode has a limited junction capacitance. Therefore, when illuminated with light for a long period of time, the PD junction capacitance will be saturated, as shown in FIG. 1(c). This problem must be considered in the sequential access method. For example, in the case where the light intensity is higher at one wavelength and lower at another, if the readout period is set longer to ensure an adequate length of time for the detection of the low-intensity light, a photocurrent in excess of the junction capacitance will flow in a photodiode receiving light at a wavelength where the light intensity is high, and eventually saturate the capacitance. A saturated device cannot serve as a detector since its signal can no longer show any readable change.
Conversely, selecting a shorter readout period to prevent the saturation decreases the amount of electric charge discharged from the photodiode corresponding to the wavelength where the light intensity is low, only to produce a faint output signal and hence decrease the S/N ratio of the measurement. Therefore, unnecessary shortening of the readout period should be avoided to ensure a high level of measurement accuracy.
Thus, in the sequential access method, it is crucial to appropriately specify the readout period for each photodiode. However, no satisfactory solution has been found to the tradeoff problem between the prevention of saturation of the photodiode and the improvement of the S/N ratio.
One conceivable method for solving the aforementioned problem is to control the signal readout operation so that the photodiodes will be accessed in descending order of the intensity of light falling on them before each photodiode is saturated. For example, the Japanese Unexamined Patent Application Publication No. H08-015013 discloses a device with a control circuit for independently controlling individual photodiodes, thus allowing the readout period to be independently specified for each photodiode based on the measured value of the background spectrum. However, this method also has several problems: For example, increasing the number of photodiodes requires more complex scheduling for reading out signals from all the photodiodes without changing the readout period for each photodiode. Another problem exists in that this method requires a randomly accessible photodiode array, which is expensive and hence unfavorable when considering the issue of cost reduction.
A technique aimed at solving the aforementioned problems within the framework of the sequential access method is disclosed in the Gazette of Japanese Utility Model Registration No. 2564792, in which an attenuation filter is provided for only the photodiodes that cover the wavelength range where the light intensity is high. Another method is shown in Japanese Unexamined Patent Application Publication No. H08-122150, which includes specifying a limited wavelength range for the measurement and automatically selecting an appropriate charging time for the specified wavelength range.
These techniques also have other problems to be solved, such as the instability of incident light falling on the photodiodes located at the boundary of the attenuation filter or the inability to perform the measurement over a wide wavelength range.
The shortest possible readout period is determined by the period of time required for the analyzing apparatus to read out signals from a single photodiode and the number of photodiodes constituting the photodiode array. Therefore, in the case of using a processor (e.g. a signal-processing circuit) with a low processing capability or a photodiode array with a large area, some of the photodiodes may be saturated even if the shortest readout period is set at the smallest value.
The discussions on the problems and measures described so far are also applicable to an opposite type of photodiode which will be charged when the photocurrent flows.
The present invention is aimed at providing a photodiode array adopting a storage readout method, which is capable of easily setting an appropriate charging time for each photodiode yet preventing the photodiodes from saturation. The present invention is also aimed at providing a signal readout method for a photodiode array.